For processing oxide materials, such as silicon dioxide for example, into a nanofiber shape, there are applicable a chemical vapor deposition method (CVD method) and a vaporized substrate deposition method (VSD method). In the CVD method, fiber-shaped SiO2 is grown in a gas phase at low pressures, using a gas including silicon (Si) as a raw material. For the raw material gas, for example, monosilane (SiH4), disilane (Si2H6), tetraethoxysilane (TEOS), or the like are used. In the CVD method, each of metal oxide nanofibers are formed as a crystallized metal oxide (single crystals or polycrystals) with a needle shape. On the other hand, in the VSD method, an Si substrate on which a catalyst is formed is fired to evaporate Si from the Si substrate, thereby forming fiber-shaped SiO2 by using the Si as the raw material. According to the VSD method, the resulting metal oxide is amorphous nanofibers and has a metal-oxide-nanofiber shape with high flexibility.
Note that Patent Literatures 1 and 2, for example, are known as prior art documents related to the present invention.
The VSD method is an excellent one because, without any expensive apparatus, it is capable of locally forming metal oxide nanofibers only at a desired place on a metal substrate. However, since atoms to be a raw material are supplied from the substrate in the VSD method, the method is limited for cases where the metal element configuring the metal oxide nanofibers is the same as that configuring the substrate. For SiO2 nanofibers, for example, the usable substrate is limited to the Si substrate. Therefore, on a substrate freely-selected, it is difficult to form metal oxide nanofibers of a kind of metal different from that of the substrate.
Moreover, the VSD method requires that the substrate be heated up to a temperature at which the material evaporates therefrom. For example, when using a Si substrate as the metal substrate, the substrate needs to be subjected to a high-temperature treatment in firing, at temperatures of not less than 1000° C. Therefore, it is difficult to form metal oxide nanofibers on an object such as a substrate of glass or plastic, a structure of glass or plastic, and a composite structure including glass and plastic, with the glass and the plastic each having a lower melting point than the above temperatures.
In contrast, in the CVD method, raw materials that configure the metal oxide nanofibers are supplied in gas form. Therefore, the metal oxide nanofibers can be formed on an object relatively freely-selected including, for example, a glass, resin, and tin-doped indium oxide (ITO) substrates which have low heat resistance in particular. However, the method generally requires expensive apparatus so as to generate gas-plasma in vacuum and to control the distribution of the plasma; therefore, it is unsuitable for applications of forming nanofibers with high uniformity on a large-area substrate.